1. Field of the Invention
The present invention relates to a dust removing apparatus for removing dust from gas exhausted from a combustor of fossil fuel such as coal, an iron manufacturing plant, a cement plant, a chemical plant and the like.
2. Description of the Prior Art
One example of a dust removing apparatus for removing dust from high-temperature gas in the prior art is shown in section in FIG. 7. In this heretofore known dust removing apparatus, a plurality of porous ceramic tubes 01 are arrayed along the direction of flow of a dust-containing gas within a casing 02 that is disposed nearly vertically. The dust-containing gas having flowed into the casing 02 through a dust-containing gas inlet 03 at the top of this casing 02 flows into the respective porous ceramic tubes 01 and has its dust removed while passing from the inside to the outside tube walls. The cleaned gas then flows out through a clean gas outlet 04. On the other hand, the dust removed from the dust-containing gas is led to a dust hopper 05 at the bottom of the casing 02 and is exhausted therefrom. It is to be noted that the top and bottom ends of the respective porous ceramic tubes 01 are respectively supported by header plates (partition plates) 06.
In the above-described heretofore known dust removing apparatus shown in FIG. 7, because dust may adhere to and accumulate on the inner tube surfaces of the ceramic tubes 01 performing the filtering action, high-pressure backwashing air is intermittently injected against the tubes 01 from the clean gas side, that is, from the outer surfaces of the ceramic tubes 01 so that the adhered dust is forced off of the inner surfaces.
However, the above-described dust removing apparatus presents a problem in that when the backwashing is effected simultaneously over the entire region as described above, a disturbance is created in the flow of the clean gas. Also dust freed from the tubes during the backwashing operation would disperse in the dust-containing gas, and hence a large proportion of the dust would have to be refiltered in the next filtering operation.
Another example of a dust removing apparatus for removing dust from high-temperature gas by making use of porous ceramic tubes in the prior art is shown in section in FIG. 8. In this dust removing apparatus, two stages (upper and lower) of a plurality of open-ended porous ceramic tubes 012 are spaced vertically apart in a casing 011. Upper and lower ends of the tubes 012 of each stage are supported, respectively, by header plates 013. The inside of the casing 011 is partitioned into an upper space 015 communicating with a gas inlet 014 at the top of the casing 011, a central space 017 communicating with a clean gas outlet 016 in the middle portion of the casing 011, and a lower space 019 communicating with a dust hopper 018 at the bottom of the casing 011. A dust-containing gas 020 such as a boiler combustion gas or the like has its dust removed while passing through the respective ceramic tubes 012 from their inside to their outside, and flows through the respective clean gas outlet pipes 016 to a clean gas pipe 021.
In addition, in order to remove the dust adhered to the ceramic tubes 012, backwashing high-pressure air is intermittently fed from a compressed air source 022 through a backwashing valve 023 and air pipes 024 and is then injected from nozzles 025 through the respective clean gas outlet pipes 016 to the central space 017 of the casing 011.
In this heretofore known dust removing apparatus shown in FIG. 8, however, because the plurality of ceramic tubes 012 are open-ended tubes and the clean gas is extracted horizontally through the ceramic tube walls, at the lowermost portion (the outlet) of the respective ceramic tubes D, the downward gas flow velocity becomes zero as shown in FIG. 9 (the ordinate positions A-D corresponds to the levels A-D in FIG. 8). Hence, dust removed from the upper portions of the ceramic tubes would stagnate there, and it was difficult to exhaust the dust to an ash treatment system (not shown) positioned downstream of the dust hopper 018. And, in an extreme case, the ceramic tubes would be clogged by the dust. Furthermore, if abnormal combustion should occur in the upstream boiler or the like, injection of the backwashing air had the potential to induce after-burning.
One example of a dust removing apparatus in the prior art which resolved the above-described problems is disclosed in Japanese Patent Publication No. 3-24251 (1991). In this dust removing apparatus, which is shown in FIG. 10, in order to insure a gas flow velocity at the lowermost portions of the respective ceramic tubes 012, a dust-containing gas circulating line 046 extends from the lower space in the casing 011 up to the upper space 015 and a circulating pump 047 is connected to this line. It is to be noted that in FIG. 10, components identical to those shown in FIG. 8 are designated by like reference numerals and a detailed explanation thereof will be omitted.
However, the heretofore known dust removing apparatus shown in FIG. 10 still is subject to the following problem. That is, a circulating pump which can withstand a temperature of the gas as high as 850.degree. C. does not actually exist at the present time and would in fact be difficult to be put into practice. And even if such a pump could be put into practice, the efficiency of the dust removing apparatus itself would be low due to the circulating of the gas.